id: Q8N4B4 gene_symbol: FBXO39 product_type: PROTEIN status: COMPLETE taxon: id: NCBITaxon:9606 label: Homo sapiens description: >- FBXO39 (F-box only protein 39) is a 442-amino-acid F-box protein that serves as the substrate-recognition component of an SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex (a Cullin-RING ligase, CRL1). It contains an N-terminal F-box domain (residues ~16-61) through which it docks onto the SKP1-CUL1 scaffold, and a C-terminal leucine-rich-repeat (LRR)/RNI-like region predicted to mediate substrate binding. As an F-box "other" (FBXO) protein, FBXO39 acts as the interchangeable specificity subunit that targets substrate proteins for poly-ubiquitination and subsequent proteasomal degradation; the catalytic RING/E2-recruiting role within the complex is provided by RBX1/CUL1, not by FBXO39 itself. FBXO39 expression is testis-enriched and the gene is a cancer/testis antigen (historically named BCP-20), aberrantly re-expressed in several tumors (colorectal, glioma/glioblastoma, others), where it has been used as a prognostic/immunotherapy-candidate biomarker; knockdown in osteosarcoma cells reduces proliferation and increases apoptosis, indicating functional relevance to tumor-cell viability. The protein remains poorly characterized at the mechanistic level: no validated physiological substrate was established in the curated literature, although an emerging report proposes that FBXO39 promotes degradation of p53 to drive LDHA-mediated aerobic glycolysis in colorectal cancer. Its subcellular localization has not been experimentally determined. existing_annotations: - term: id: GO:0019005 label: SCF ubiquitin ligase complex evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: part_of review: summary: Phylogenetic (PAN-GO) assignment that FBXO39 is part of an SCF E3 ubiquitin ligase complex, consistent with its F-box domain and the UniProt-documented SKP1/CUL1 interaction. This is the core cellular-component assignment for an F-box protein. action: ACCEPT reason: Core localization/complex membership. FBXO39 has an F-box domain and is annotated by UniProt as a substrate-recognition component of the SCF complex that directly interacts with SKP1 and CUL1; ComplexPortal also defines an SCF complex variant containing FBXO39 (CPX-7979). supported_by: - reference_id: file:human/FBXO39/FBXO39-uniprot.txt supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex. - term: id: GO:0031146 label: SCF-dependent proteasomal ubiquitin-dependent protein catabolic process evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: involved_in review: summary: Phylogenetic (PAN-GO) assignment that FBXO39 participates in SCF-dependent proteasomal protein catabolism, the core biological process for an SCF substrate-recognition subunit. action: ACCEPT reason: Core biological process. As an SCF substrate-recognition component, FBXO39's function is to target substrates for SCF/proteasome-mediated degradation, consistent with the F-box domain and the Ubl conjugation pathway keyword in UniProt, and with the functional placement of FBXO39 as an F-box/SCF (CRL1) substrate-recognition receptor. Falcon-sourced literature supports this at the family level but confirms no validated FBXO39 substrate in the curated window (an emerging 2026 p53-degradation mechanism aside), so this remains an inferred core process rather than a substrate-specific one. supported_by: - reference_id: file:human/FBXO39/FBXO39-uniprot.txt supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex. - reference_id: file:human/FBXO39/FBXO39-deep-research-falcon.md supporting_text: This is strongly supported as a *family-level mechanism* (xuan2024theemergingand pages 1-2, wang2014rolesoffbox pages 1-3), but **FBXO39-specific substrate repertoire remains poorly established** in the retrieved 2023–2024 literature. - term: id: GO:0005515 label: protein binding evidence_type: IPI original_reference_id: PMID:32296183 qualifier: enables review: summary: IntAct binary interaction (HuRI reference interactome) between FBXO39 (Q8N4B4) and MEOX2 (Q6FHY5). The bare protein binding term is uninformative on its own; the interaction is recorded in the UniProt INTERACTION block. action: KEEP_AS_NON_CORE reason: Records a real binary interaction (FBXO39-MEOX2) from a high-throughput interactome, but bare protein binding is uninformative per curation guidelines and is not a core function. MEOX2 is a plausible candidate substrate but this has not been functionally validated. supported_by: - reference_id: file:human/FBXO39/FBXO39-uniprot.txt supporting_text: 'Q8N4B4; Q6FHY5: MEOX2; NbExp=3; IntAct=EBI-12383478, EBI-16439278;' - term: id: GO:0019005 label: SCF ubiquitin ligase complex evidence_type: NAS original_reference_id: PMID:34445249 qualifier: part_of review: summary: ComplexPortal NAS assignment (from a review of the SCF complex family) that FBXO39 is part of an SCF E3 ubiquitin ligase complex. Redundant with, and consistent with, the IBA assignment and UniProt FUNCTION. action: ACCEPT reason: Core complex membership; corroborated by UniProt (substrate-recognition component; directly interacts with SKP1 and CUL1) and ComplexPortal CPX-7979. The cited review establishes that variable F-box proteins such as FBXO39 form SCF complexes that determine substrate specificity. supported_by: - reference_id: PMID:34445249 supporting_text: These SCF complexes are distinguishable by variable F-box proteins, which determine substrate specificity. - term: id: GO:0031146 label: SCF-dependent proteasomal ubiquitin-dependent protein catabolic process evidence_type: NAS original_reference_id: PMID:34445249 qualifier: involved_in review: summary: ComplexPortal NAS assignment (from the SCF-family review) that FBXO39 participates in SCF-dependent proteasomal protein catabolism. Redundant with the IBA assignment. action: ACCEPT reason: Core biological process; the cited review states that SCF E3 ligase complexes modify substrates with poly-ubiquitin chains to target them for proteasomal degradation, the canonical role of an F-box substrate-recognition subunit. supported_by: - reference_id: PMID:34445249 supporting_text: encompasses a group of 69 SCF E3 ubiquitin ligase complexes that primarily modify protein substrates with poly-ubiquitin chains to target them for proteasomal degradation references: - id: GO_REF:0000033 title: Annotation inferences using phylogenetic trees findings: [] - id: PMID:32296183 title: A reference map of the human binary protein interactome. findings: - statement: HuRI all-by-all binary interactome; the FBXO39 (Q8N4B4) interaction with MEOX2 (Q6FHY5) derives from this systematic yeast two-hybrid map. reference_section_type: ABSTRACT reference_review: relevance: LOW correctness: VERIFIED review_notes: High-throughput binary interactome (HuRI). Source of the bare GO:0005515 protein binding annotation (FBXO39-MEOX2). Interaction is also recorded in the UniProt INTERACTION block; informative as a candidate-substrate lead but not a validated function. - id: PMID:34445249 title: The SCF Complex Is Essential to Maintain Genome and Chromosome Stability. findings: - statement: Review establishing that the SCF (SKP1-CUL1-F-box protein) family comprises ~69 E3 ligase complexes whose interchangeable F-box proteins determine substrate specificity and target substrates for proteasomal degradation. reference_section_type: ABSTRACT reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: PubMed-verified review (Int J Mol Sci 2021). Cited by ComplexPortal as NAS basis for FBXO39 SCF complex membership and SCF-dependent catabolic process. Function-level support is family/complex-level, not FBXO39-specific experimental data; abstract-only in cache. - id: file:human/FBXO39/FBXO39-deep-research-falcon.md title: Falcon deep research report for human FBXO39 findings: - statement: FBXO39 is inferred to act as a substrate adaptor of an SCF/CRL1 E3 ligase from its F-box+LRR architecture, but its FBXO39-specific substrate repertoire remains poorly established and no validated substrate was reported in the curated literature. supporting_text: This is strongly supported as a *family-level mechanism* (xuan2024theemergingand pages 1-2, wang2014rolesoffbox pages 1-3), but **FBXO39-specific substrate repertoire remains poorly established** in the retrieved 2023–2024 literature. - statement: FBXO39 is a cancer/testis antigen (BCP-20), testis-enriched and aberrantly re-expressed in tumors, and its knockdown in osteosarcoma cells reduces proliferation and increases apoptosis. supporting_text: lentiviral shRNA knockdown of FBXO39 reduced proliferation (Celigo-based counts; MTT assays) and increased apoptosis (Annexin V flow cytometry; caspase 3/7 activity) - statement: An emerging (2026) report proposes FBXO39 promotes colorectal cancer progression and LDHA-mediated aerobic glycolysis via p53 degradation, supported by co-IP and ubiquitination assays, but this is outside the curated window and treated as emerging. supporting_text: FBXO39 promotes colorectal cancer progression and LDHA-mediated aerobic glycolysis via **p53 degradation** reference_review: relevance: MEDIUM correctness: UNVERIFIED review_notes: >- Falcon synthesis anchored on Zheng et al. 2018 (osteosarcoma knockdown), Motalebzadeh et al. 2018 (CRC CTA/BCP-20), Azimi et al. 2025 and Zhuo et al. 2024 (glioma CTA biomarker), Xuan et al. 2024 (testis expression), and an emerging 2026 colorectal-cancer p53/LDHA mechanism (Liu et al.). The CTA identity and cancer-biomarker context are well supported; the p53-substrate mechanism is a single emerging (2026) lead. Falcon cites author-year/DOIs not PMIDs and the papers are not in the local cache, so claims are treated as leads (UNVERIFIED) and not promoted to new substrate-specific GO terms. core_functions: - description: Substrate-recognition (specificity) component of an SCF (SKP1-CUL1-F-box protein) / CRL1 E3 ubiquitin ligase complex; via its F-box domain it docks onto SKP1-CUL1 and recruits substrate proteins for poly-ubiquitination, committing them to SCF-dependent proteasomal degradation. The catalytic E2-recruiting activity is provided by the RBX1/CUL1 core, while FBXO39 functions as a ubiquitin-like ligase-substrate adaptor. molecular_function: id: GO:1990756 label: ubiquitin-like ligase-substrate adaptor activity locations: - id: GO:0005737 label: cytoplasm supported_by: - reference_id: file:human/FBXO39/FBXO39-uniprot.txt supporting_text: Substrate-recognition component of the SCF (SKP1-CUL1-F-box protein)-type E3 ubiquitin ligase complex. - reference_id: file:human/FBXO39/FBXO39-uniprot.txt supporting_text: Directly interacts with SKP1 and CUL1. directly_involved_in: - id: GO:0031146 label: SCF-dependent proteasomal ubiquitin-dependent protein catabolic process proposed_new_terms: [] suggested_questions: - question: What is the physiological substrate (or substrates) recognized by the FBXO39 LRR/C-terminal region, and are the candidate targets (MEOX2 from HuRI; p53 from the emerging colorectal-cancer report) bona fide direct SCF(FBXO39) ubiquitination substrates? - question: Given the testis-enriched expression of FBXO39, does it have a specific role in spermatogenesis or germ-cell protein turnover, and how does this reconcile with its cancer/testis-antigen (BCP-20) re-expression and pro-tumorigenic phenotypes in cancer? - question: Where does FBXO39 localize, and does its reported pro-proliferative/anti-apoptotic effect in tumor cells operate through SCF-dependent degradation of a growth-suppressive substrate (e.g. p53)? suggested_experiments: - description: Reconstitute the SCF(FBXO39) complex (SKP1-CUL1-RBX1-FBXO39) in vitro and perform ubiquitination assays with candidate substrates (MEOX2, p53) to confirm substrate-receptor activity and identify ubiquitin-chain topology. - description: Perform affinity-purification/proximity-labeling mass spectrometry of tagged FBXO39 in testis-derived or germ-cell models and in cancer cells to define the SCF(FBXO39) interactome and candidate substrate repertoire, validated by stabilization upon FBXO39 knockout. - description: Determine FBXO39 subcellular localization (immunofluorescence, fractionation) and test whether its tumor-cell proliferation/apoptosis phenotype requires intact SCF assembly (F-box mutants) and degradation of specific substrates such as p53.